1. Field of the Invention:
The present invention relates to the field of three dimensional displays.
2. Prior Art:
Three dimensional displays utilizing vibrating mirrors are known in the prior art. In accordance with this technology, a viewer does not look directly at the cathode ray tube (CRT) but instead looks at a mirror to see the reflection of the CRT image from a CRT mounted over the observer. When the mirror is flat, of course, the CRT image appears to be at a distance equal to the distance from the observer to the mirror plus the distance from the mirror to the CRT. By vibrating the mirror around its normally flat condition so that the mirror vibrates through concave and convex conditions, the image on the CRT will be made to appear further and closer away from the observer, respectively. The basic concept of such three dimensional displays is that if one vibrates the mirror at a relatively low frequency, say 30 Hz., and displays data at a much higher rate, so that multiple images, properly proportioned etc. can be displayed during any stroke of the mirror, those multiple images will appear to the observer as providing a single three dimensional image. Normally, the mirror is vibrated sinusoidally, as significant deviation from sinusoidal will give rise to substantial harmonics in the audible range, to the distraction of the observer. Further, sufficient deviation from sinusoidal vibration to attempt to obtain a more linear sweep through the display volume is generally not practically achieved, and accordingly not normally attempted.
As mentioned before, the timing of the two dimensional images on the CRT is coordinated with the position of the mirror so that individual, apparently equally spaced and properly sized planes of the three dimensional image are reflected by the mirror during the mirror forestroke, and interlaced images are reflected during the mirror backstroke motion. However, because the mirror velocity is sinusoidal rather than constant and for that matter the apparent depth of any image is not a linear function of mirror position, images put up at a uniform rate will appear to be concentrated toward the ends of the display volume because of the very low mirror velocity and attendant low rate of sweep through the display volume near the ends of the mirror travel. The desired three dimensional image is therefore created by putting up the images at a non-uniform time rate coordinated with mirror velocity so that the images appear equally spaced throughout the display volume during any stroke of the mirror. Further, the resolution in the third demension may be improved if during the opposite stroke, additional equally spaced images are put up with a timing so as to be interlaced with those put up during the prior stroke, so as to give the appearance of twice as many equally spaced images throughout the display volume.
The foregoing requirements for the apparent equal spacing of the images in the third dimension throughout the display volume by the interlacing of the forestroke and backstroke images requires both accurate knowledge of the mirror motion and careful timing of the display coordinated with that mirror motion. Various types of mirror sensors can be used although the requirements of accuracy and absence of drift impose severe requirements on the mirror's sensing system. Also the control of the display timing with respect to mirror motion may be done under software control, although doing this function under software control puts a heavy burden on the graphics computer, already sufficiently occupied with other functions such as image generation, coordinate transformations for image rotation, communication with the host computer, etc. The present invention comprises a particularly simple system for accurate tracking of mirror vibration in controlling the display rate with velocity without complicated sensing systems or overhead burden on the graphics computer.